1. Field of the Invention
The present invention relates to measures to prevent leakage of electromagnetic wave noise in a plasma ignition system, which is used for ignition in an internal combustion engine.
2. Description of Related Art
Recently, from a standpoint of environmental protection, lean mixture combustion or supercharged mixture combustion, for example, is required in an internal combustion engine to reduce emissions in combustion exhaust gas or to improve fuel mileage, so that an ignition condition is becoming severe. Accordingly, an ignition system, in which stable ignitionability is achieved, is required in an engine of poor ignitionability.
In the case of ignition of the engine, an ignition system using an ordinary spark plug 10z shown in FIG. 10A includes a battery 31z, an ignition switch 32z, an ignition coil 33z, an electronic control unit (ECU) 35z, an ignition coil drive circuit (transistor) 34z, a rectifying device 21z, and the spark plug 10z. As shown in FIG. 10B, when the ignition switch 32z is thrown, a primary voltage having a low voltage is applied to a primary coil 331z of an ignition coil 33z from the battery 31z in response to an ignition signal from the ECU 35z. Subsequently, when the primary voltage is cut off through the switching of the ignition coil drive circuit 34z, a magnetic field in the ignition coil 33z changes, and thereby a secondary voltage in a range of −10 to −30 kV is generated in a secondary coil 332z of the ignition coil 33z. As a result, electric discharge takes place in a center electrode 110z and a ground electrode 131z, and accordingly a high-temperature region is generated in a small area. In the case of the ignition by the ordinary spark plug 10z, the above high-temperature region serves as a source of ignition to excite ignition and explosion of a compressed air-fuel mixture. Meanwhile, a current of about 35 mA rectified through a diode 21z passes through the secondary coil 332z during a conducting period of about 2 ms, which is a relatively long duration, and energy of about 35 mJ is released to the spark plug 10z. 
In the case of ignition by a plasma ignition system 1x shown in FIG. 12A, when an ignition switch 31x is thrown (see FIG. 12B), a primary voltage having a low voltage is applied to a primary coil 321 of an ignition coil 32x from a discharge battery 30x. By switching of an ignition coil drive circuit (transistor) 33x controlled by an electronic control unit (ECU) 34x, the primary voltage is cut off and thereby a magnetic field in the ignition coil 32x changes. Consequently, a secondary voltage in a range of −10 to −30 xV is generated in a secondary coil 322x of the ignition coil 32x. The insulation in a discharge space 140x breaks down and electric discharge is started when the secondary voltage reaches a discharge voltage proportional to a discharging gap in the discharge space 140x formed between a center electrode 110x and a ground electrode 130x. Meanwhile, energy (e.g., −450V, 120 A) stored in a capacitor 42x from a plasma energy supply battery 40x, which is provided separately from the discharge battery 30x, is released to the discharge space 140x at once. Accordingly, gas in the discharge space 140x enters into a high-temperature and pressure plasma state, and is injected through an opening 132x formed at a leading end of the discharge space 140x. As a result, a very high temperature range in a range of thousands to tens of thousands of degrees Celsius and having great directivity is generated in a wide range of volume. Thus, such a plasma ignition system is expected to be applied to an ignition system in an internal combustion engine of difficult ignitionability in which lean mixture combustion or supercharged mixture combustion, for example, is performed. In addition, when the plasma ignition system is applied to the ordinary spark plug, plasma having high energy is generated between electrodes of the plug. Therefore, improvement in ignitionability is expected.
However, in the conventional plasma ignition system Ix, the energy stored in the capacitor 42x for plasma generation is instantaneously supplied to a plasma ignition plug 10x. Consequently, as shown in FIG. 12B, a high current of about 120 A is passed for a conducting period of about 8 μsec, which is an extremely short duration. Since the above passing of high current is periodically repeated according to rotation of the engine, an electromagnetic wave noise of high frequency is generated. Malfunction of the electronic control unit installed in a vehicle or the like is caused by such an electromagnetic wave noise, and as a result, an accidental fire of the engine may be caused. As a method for preventing the above electromagnetic wave noise, a method for blocking the electromagnetic wave noise is disclosed in JP55-172659U corresponding to U.S. Pat. No. 4,327,702. The electromagnetic wave noise is blocked, by using a shielding wire for a wiring for plasma generation connecting a plasma generation power source and a plug, giving an electromagnetic wave shield to cover the whole plug, and using a resistance wire for a wiring for electric discharge connecting an electric discharge power source and the plug.
Nevertheless, the internal combustion engine such as a car motor usually includes a plurality of cylinders, and accordingly, the electromagnetic wave shield needs to be given over a very wide range when the conventional method illustrated in JP55-172659U is employed. In a plasma ignition system, in which a plurality of plasma ignition plugs 10x (1), 10x (2), 10x (3), 10x (4) is connected to an ignition coil 32x via a distributor 60x, as shown in FIG. 11, when a shielding wire is used for a plasma generation wiring 400x connected to each plug, the whole plug is covered with an electromagnetic wave shield, and a resistance wire 36x is used for a high voltage supply wiring, in order to restrict the generation of the electromagnetic wave noise, stray capacitances Cs (1 to 6) in electromagnetic wave shield parts Sd (1 to 6) are not constant since the length of each shielding wire differs. Accordingly, it is difficult to maintain an earth potential of each electromagnetic wave shield part at the same electric potential, and thereby an electric potential difference is generated between the electromagnetic wave shields. Such an electric potential difference serves as a generation source of a new electromagnetic wave noise. Also, electric field concentration is generated in a connection part of each electromagnetic wave shield part, and it is difficult to block the electromagnetic wave noise completely.
In addition, a transmit circuit is formed from the ignition coil 32x and the plasma ignition plug 10x as a discharging space. When high voltage is applied from the ignition coil 32x and electric discharge is started, the electromagnetic wave noise is generated and may leak to the outside because a plasma generation wiring connecting a center-electrode terminal area 112x and the capacitor 42x for plasma generation serves as an antenna. In the ordinary spark plug, such transmission of the electromagnetic wave noise is prevented by interposing a resistance element between the ignition coil and the plug. However, as mentioned above, the high current must be passed through the plasma generation wiring. Thus, the electromagnetic wave noise at the time of starting of the electric discharge cannot be absorbed by interposing the resistance element on the plasma generation wiring.